People are generally aware of the most common causes of hallucinations, like schizophrenia and a really high fever. But lesser known are the more rare causes; here are six you probably haven't heard about.

Changing the composition of your gut bacteria may alter mood, anxiety, stress, and pain sensitivity, suggests a new study on the effects of eating probiotic yogurt on the brain activity of healthy women. NBC / Saturday Night Live

A new study from the University of California, Los Angeles may explain why women in yogurt commercials are always so happy. It's known that eating probiotic yogurt can change the makeup of your gut bacteria for the better, but researchers recently discovered that it may alter your brain function as well. These findings could lead the way to creating probiotic dietary interventions that alter mood, anxiety, stress, and pain sensitivity.

Symbiotic gut bacteria, the complex ecosystem of microorganisms that live in the human digestive system, promote health benefits by strengthening immunity, helping us digest food, as well as maintain a healthy weight and blood pressure.

Previousstudies on rodents suggest that dietary changes to gut bacteria altered the rats' emotional behavior and pain sensitivity. But until now there has been little evidence that the bacteria eaten in food can affect human brain function.

In a recent study of healthy women published in the journal Gastroenterology, researchers led by Dr. Kirsten Tillisch of UCLA's School of Medicine found that those who ate probiotic yogurt for a month showed altered brain function, both in resting brain activity and in response to an emotional attention task.

"Our findings indicate that some of the contents of yogurt may actually change the way our brain responds to the environment. When we consider the implications of this work, the old sayings 'you are what you eat' and 'gut feelings' take on new meaning," said Tillisch in a news release.

"Time and time again, we hear from patients that they never felt depressed or anxious until they started experiencing problems with their gut. Our study shows that the gut-brain connection is a two-way street."

How Does Gut Bacteria Change Brain Function?

To investigate the effects of probiotics on brain function, Tillisch's team recruited 36 women between the ages of 18 and 53 who had a healthy average body mass index, averaging 23 kg/m2.

The women were randomly assigned to one of three groups: one that ate a yogurt with live bacterial cultures (containing probiotic strains like Bifidobacterium animalis, Streptococcus thermophiles, and Lactobacillus bulgaricus) twice a day for one month, another that ate a dairy product which contained no living bacteria, and another that was given no dairy products.

Before and after the one-month study period, the researchers conducted functional magnetic resonance imaging (fMRI) brain scans on the women. In each session, they started with a five-minute scan of the brain at rest, while the women lied still with their eyes closed.

Afterward, the participants went through an "emotional faces attention task," in which their brains were scanned while they matched a series of angry or fearful faces on a computer screen to other faces that appeared.

The results showed that during the emotional task, women who ate the probiotic yogurt had reduced activity in a brain network that included the somatosensory cortex, which receives sensory information, and the insula, a brain region that integrates sensory feedback from internal parts of the body including the gut. They also had reduced activity in the prefrontal cortex, precuneus, and basal ganglia, which handle aspects of cognition and emotion.

The women who ate non-probiotic yogurt or no dairy showed either no change, or an increase of activity in this network over time.

In the resting state, the brain scans of the women who ate probiotic yogurt showed stronger connectivity in a neural network which connects the periaqueductal grey (PAG) &mdash a region of the brainstem involved in responding to pain and emotional stimuli — to areas of the prefrontal cortex related to aspects of cognition like decision-making.

The women who ate no dairy, however, had stronger connectivity of the PAG to sensory and emotion-related parts of the brain, like the insula, somatosensory cortex, and amygdala.

Mood-Altering Probiotic Yogurt Might Treat Stress, Anxiety

The mechanisms behind these changes are unclear, wrote the researchers, though it's clear that gut bacteria send molecular signals to the brain that can change over time.

Dr. Emeran Mayer, the paper's senior author, explained that what we eat can alter the way our gut bacteria break down food. While diets high in vegetables and fiber promote healthy gut bacteria, the typical Western diet full of fats, sugars, and carbohydrates, all of which can lead to a completely different set of less helpfulgut bacteria.

"Now we know that this has an effect not only on the metabolism but also affects brain function," he said.

In future studies, the research team hopes to identify which molecular signals from the gut bacteria lead to shifts in brain activity. People with digestive conditions linked to gut dysbiosis, or imbalances in gut bacteria, like irritable bowel syndrome, might show such shifts in brain response as they are treated with probiotics.

"If confirmed, modulation of the gut flora can provide novel targets for the treatment of patients with abnormal pain and stress responses associated with gut dysbiosis," the researchers write.

Mayer also suggested that specific probiotic strains in yogurt could have health benefits like relieving anxiety, stress, and other mood symptoms over time.

As tests to analyze individual microbiomes become more readily available, it will become easier to see how someone's gut bacteria makeup influences factors like brain development, stress, and pain sensitivity.

It's possible that changing the composition of gut bacteria could lead to treatments for chronic pain disorders, he said, as well as symptoms of brain conditions like autism, Parkinson's, and Alzheimer's disease.